1. Field of the Invention
The invention is concerned with a process for the delignification of cellulose fiber raw materials, specifically wood chips, using alcohol.
2. Description of the Related Technology
In the search for environmentally benign processes for the production of pulp, the so-called organosolv processes have increasingly gained interest. Organosolv processes are those in which alcohol is used for the purpose of delignification of the fiber raw materials. These processes do not use sulfur compounds which are today's dominant pulping chemicals.
In the most commonly used process today, the Kraft process, sulfide is used which causes environmentally unacceptable air emissions. It is also the cause of nuisance due to odor.
The sulfite process is somewhat easier to control than the Kraft process; however, the fiber properties of sulfite pulps are inferior to Kraft pulps.
The organosolv process, using a mixture of water and alcohol (e.g., EU PS0090969), made it possible to produce pulps of acceptable quality without the disadvantages of the sulfur compounds.
Pulps of very good fiber properties were produced by means of a two-stage process in which the wood chips were first cooked in a mixture of alcohol and water under acid conditions, to be followed by a second stage in which sodium hydroxide and alcohol were added to the aqueous alcohol solution and the cook continued under alkaline conditions. Prior to the cooking stages was a separate impregnation stage in which the wood chips were first pre-steamed and subsequently impregnated at low temperature with an aqueous alcohol solution.
It was also proposed to pulp cellulose fiber raw material with alcohol together with sodium hydroxide. Trial cooks in batch digesters showed, however, that the delignification of the fiber raw material was unsatisfactory and the KAPPA numbers of the pulps obtained were quite high despite long cooking times.